The operation of wireless communication systems at ever higher frequencies is desired because of higher data rates and capacities achievable with higher frequency and available bandwidths. Thus, millimeter wave (MMW) systems (e.g., operating at 20-200 GHz) are addressing wireless communication needs and becoming increasingly used. Such MMW systems require guided-wave structures in order to route signals efficiently. These guided-wave structures include various planar implementations (coplanar waveguide, microstrip, stripline) and machined waveguides. Several factors influence the choice of guided-wave structure, including the ultimate performance and the cost.
Planar guided-wave approaches are very attractive because they can be realized using low cost processes. Although it is possible to utilize planar structures to make passive elements such as diplexers, filters, and resonators, the resulting devices typically are limited in performance. The reason for the limited performance can be seen when one compares a property of resonators called the Intrinsic Quality Factor, or Q, and the ultimate performance of components in terms of loss. Q is a ratio of total energy stored in a resonator and the loss per resonant cycle. Q is affected by multiple physical factors such as resonator shape and surface quality that affect how currents flow in the resonator, how energy can radiate and thus is an intrinsic measure of factors influencing loss and efficiency. Performance for any given passive structure configuration, such as a filter, improves with higher Q. Sharper filter roll-off requires greater numbers of resonators, and as the number of resonators in a structure increases (typically ranging from 2 to 10 resonators), so does the insertion loss. Also, the narrower the bandwidth of the filter as a percentage of the operational frequency, the greater is its insertion loss. The greater the Q, the less these loss imposing factors diminish performance.
Planar elements, with Q factors in the 100-250 range, are adequate for many MMW system components such as image reject filters, or other filters with large bandwidths. Efforts have been made to create quasi-planar MEMS-based filters on membranes because an air dielectric has very low loss, approaching that of vacuum. Filters of this variety have demonstrated Q factors nearing 400. On the other hand, machined waveguide elements have achieved Q numbers more than an order of magnitude higher than those of the planar structures. As a result, machined waveguides are commonly found in MMW systems. Unfortunately, these machined waveguides are usually individually fabricated from metals, which add weight and expense to the system. Additionally, the necessary accuracy of these structures adds to their expense, either because of costly precision machining steps for each element, or labor content required to compensate for machining inaccuracies with tuning screws or other adjustment means. Thus, there is a need for performance levels achievable with machined waveguide, but without the cost penalties associated with typical fabrication techniques for these components, and preferably without the considerable weight penalty of metallic materials.
Others have recognized these needs and have sought to reduce cost through molding techniques. The machining accuracy necessary to produce a single nearly-perfect waveguide element could in concept be employed to make a complementary mold tool and the resultant molded part would repeat the desired dimensions if it can be consistently processed. Many suitable materials, including metals, ceramics, and plastics can be cast and/or molded.
MMW components are typically used over the range of −55 to 125° C. They often are used in combination with soldered components that are processed in a typical range of 210-260° C. for short periods of time. In order to have consistent electrical characteristics, MMW waveguide components should exhibit very low thermal and mechanical hysteresis and thus should be operated well below the softening point (or glass transition temperature) of their constituent materials and should have high flexural strength and high elastic modulus. Another desirable characteristic is for the thermal expansion characteristic to be relatively low, both for minimal thermal impact over the range of operation, and to minimize stresses at material interfaces and mating surfaces with other components. Many metals and ceramics exhibit the desired characteristics, but very few plastics do so. Despite this, plastics remain very attractive for weight and cost reasons. Thus, there is a need for advantages that can be realized with plastic materials, while maintaining thermal and mechanical stability of ceramics and metals.
The vast majority of reported approaches have exploited thermoplastic materials. Thermoplastics are attractive for reasons of material reuse and process simplicity, but have several drawbacks. Most thermoplastics are high molecular weight materials whose side chains interact through intermolecular forces that diminish with sufficient heating and reform spontaneously upon cooling. This is the property that permits ease of reuse. Thermoplastics are thus processed above their glass transition temperature and near their respective melt temperatures. Typical techniques for forming parts from thermoplastics include injection molding of melted material or hot embossing of heat softened material. In order to achieve thermal stability adequate to the MMW application, melt temperature should be higher than 230° C. By this criterion, candidate thermoplastics include (Tm/Tg): PET-PBT (230-260° C./70-80° C.), polycarbonate (265° C./150° C.), PEEK (340° C./143° C.), PTFE (327° C./−97° C.) and polystyrene (240° C./100° C.), among others. At temperatures exceeding the glass transition temperature, intermolecular forces diminish and the polymer typically begins to soften and expand substantially with temperature. This property leads to undesirable stresses on the resultant parts, and creep which creates thermo-mechanical hysteresis. The result is that parts operated too far above glass transition for these thermoplastic materials distorts in ways that cause drift in the geometry of the part and the consequent electrical properties. PEEK and polycarbonate would seem to be candidates by this latter criterion. These materials have thermal expansion coefficients of 30-50 ppm/° C. and 65 ppm/° C., respectively. This means that over the operational temperature range of the parts fabricated from these materials, they will expand by a factor of 2-3× more than typical metal coatings applied to the parts (11-22 ppm/° C.). This again is a source of thermo-mechanical hysteresis, and a source of reliability issues such as metal delamination.
Mechanical properties of thermoplastics can be adjusted through the use of fillers, including flakes, spheroids, and fibers of other stiff materials such as glass and carbon, limiting the thermal expansion of the material. Nonetheless, the materials still soften and deform above the glass transition. Non-spherical fillers such as fibers become oriented during the flow of the molding process. This anisotropic orientation leads in-turn to unpredictable anisotropic thermal expansion characteristics. This effect is very clearly seen in glass fabric-reinforced printed circuit board materials, which have in-plane thermal expansion of 11-14 ppm/° C., and normal axis expansion ranging from 35-46 ppm/° C. When the fiber orientation is not closely controlled, temperature excursions result in bend and warpage of resultant parts that are unrepeatable, with consequent implications for utility of such parts in a MMW system.
Thus, there is a need for plastic parts with formation temperature <250° C., glass transition in excess of 160° C., thermal expansion coefficient matching those of typical metal coatings or mating parts, isotropic thermal expansion, high rigidity, and excellent dimensional control to eliminate the need for post-processing or adjustment. A solution with these properties and low-cost is needed to address the MMW waveguide component application.
Thermoset plastic materials are reactive in nature. This means that they form highly cross-linked networks that allow them to be quite stiff and mechanically stable. The materials are often found in liquid form, reacted upon mixing, and once cured may not be remelted. This irreversible process is often seen as a deficit for thermoset materials, as they are difficult to rework or recycle. Another type of thermoset material is found commonly in the semiconductor industry to create plastic encapsulated microcircuits. Such materials are unreacted solid mixtures, typically in pellet form, heated to a temperature at which they will flow, injected into the heated mold through a sprue and runner system, reacted in the mold at elevated temperature, then removed from the mold once partially cured. The temperature of formation is typically substantially lower than the limit of operation for the material. The process is known as transfer molding, and has very high reproduction accuracy. Because of the reactive nature of the thermoset material, adhesion to metals and fillers can be quite good, as is evidenced by the ubiquity of these materials for the aforementioned semiconductor packaging application.
Thermoset materials have been considered for the MMW waveguide component application. For example, thermoset materials have been used to join and seal multiple thermoplastic parts. Furthermore, castable materials may be used to form a matrix into which various metal and ceramic waveguide components are embedded in order to create a single assembled hybrid structure wherein thermoset plastics were one possibility for the exterior casting. However, none of the present systems and method have used thermoset plastic to manufacture MMW components and it is to this end that the disclosure is directed.